Physical shipment of materials, including the shipment of mail, merchandise, raw materials, and other goods, is an integral part of any economy. Typically, the materials are shipped in a type of shipping container or cargo box. Such containers or boxes include semi-trailers, large trucks, and rail cars as well as inter-modal containers that can be carried on container ships or cargo planes. However, such shipping or cargo containers can be used for illegal transportation of contraband such as nuclear and radioactive materials. Detection of these threats require a rapid, safe and accurate inspection system for determining the presence of hidden nuclear materials, especially at state and national borders, along with transit points such as airports and shipping ports.
Currently, both passive and active detection techniques are employed for the detection of concealed nuclear materials. Passive detection techniques are based on the principle that nuclear and radiological threats emit gamma radiation and, in some cases, neutron radiation both of which can be detected.
Active detection techniques, such as Differential Die-away Analysis (DDAA) and measurements of delayed gamma-rays and neutrons following either neutron- or photon-induced fission can be used to detect the presence of fissile materials. The radiation is measured with neutron and gamma-ray detectors, preferentially insensitive to each other's radiation. Detection of delayed neutrons is an unequivocal method to detect fissile materials compared to delayed gamma rays. However, because the number of delayed neutrons is two orders of magnitude lower than the number of delayed gamma rays, efficient and large area detectors are required for best sensitivity in neutron detection.
The most commonly used neutron detector is a Helium-3 (He-3) gas proportional chamber. In this system, He-3 interacts with a neutron to produce triton and proton ions. These ions are accelerated in the electric field of the detector to the point that they become sufficiently energetic to cause ionization of gas atoms. In a controlled environment, an avalanche breakdown of the gas can be generated, which results in a measurable current pulse at the output of the detector. By pressurizing the gas, the probability of a passing thermal neutron interacting in the gas can be increased to a reasonable level. However, He-3 is a relatively scarce material and it does not occur naturally. This makes the availability and future supply of such detectors somewhat uncertain. Further, a special permit is required to transport pressurized He-3 tubes, which can be cumbersome and potentially problematic.
The most common globally deployed passive radioactive material detectors employ a neutron moderator with one or more 2 inch He-3 detector tubes. For DDAA applications, wherein there is a need for a fast-time response, currently available detectors consist of many smaller diameter He-3 tubes. However, as described above, currently available neutron detectors have design complexities and require materials which are scarce in nature, such as He-3, which makes it difficult to develop large area and high efficiency detectors.
Currently available active detection systems also suffer from some drawbacks. In particular, these devices generally utilize accelerators that produce high energy neutrons with a broad spectrum of energies. The absorption/scattering of neutrons traveling at specific energies is difficult to detect given the large number of neutrons that pass through the object without interaction. Thus, the “fingerprint” generated from these devices is extremely small, difficult to analyze, and often leads to significant numbers of false positive or false negative test results.
In addition, conventional detection systems have limitations in their design and method that prohibit them from achieving low radiation doses, which poses a risk to the personnel involved in inspection as well as to the environment, or reduces the statistical accuracy of the detection procedure, which are prerequisites for commercial acceptance.
FIG. 1A and FIG. 1B depict conventional neutron and gamma ray detection systems. As shown in FIG. 1A, the most common globally deployed passive radioactive material detectors employ: a neutron moderator 105 having a plurality of He-3 detector tubes 116 embedded therein and covered by a lead shield 108 to attenuate background gamma-rays impinging from the back of the detector; and, a portion comprising a plastic scintillator 110 with a photo multiplier tube (PMT) 115 to detect gamma rays. The plastic scintillator 110 also functions as a moderator.
This detector configuration employs the scarce element He-3. Another commonly deployed detector wherein the gamma-ray and neutron detectors are separate is shown in FIG. 1B. Referring to FIG. 1B, the neutron moderator 105, comprising a plurality of He-3 detector tubes 116, is positioned adjacent to plastic scintillator 110, which comprises a PMT 115 and a lead shield 108. This detector configuration, however, also employs the scarce He-3 and takes up a larger footprint.
Several alternative detectors to replace He-3 detectors have been identified and/or fabricated. One solution is a system using boron-10 (B10) lined proportional counter tubes. However, in this case, large size detectors are needed to produce similar efficiency as is available in He-3 based detectors. The large size also results in long response times making these detectors not well suited for DDAA applications. Variants of these detectors include thin-walled straws lined with enriched boron carbide (10B4C). Although these detectors can produce fast response times, there is a need for hundreds of straws increasing the complexity and cost of the detector.
Other alternative neutron detectors include those based on lithium-6 (6Li) fibers; however, these require pulse-shape discrimination (PSD) to reduce the gamma-ray background, which makes the detector design significantly more complex and expensive. Also, these detectors are not well suited for fast-timing applications.
There are also neutron detectors fabricated from parallel plates coated with B10 containing materials. However, these detectors have limitations that result in making the detector unit large, requiring tiling, thus making the system complex and difficult to assemble.
While the use of both passive and active detection techniques is desirable, what is needed is a neutron and gamma-ray based detection system and method that is cost-effective, compact, and wherein the neutron detector can be fabricated in various sizes, from small to large areas from readily available materials.
There is also a need for large-area detectors for passive applications for the detection of radioactive and special nuclear materials in stationary installations for fast moving objects such as trains or for mobile-installed detectors. Further, there is a need to increase detection efficiency and coverage for DDAA applications where special nuclear material (SNM) is hidden deep in cargo.
Additionally, it would be useful to measure neutrons and gamma rays simultaneously with muon detection to increase the statistical accuracy of the passive measurements. Therefore, there is also a requirement for integrated detection systems which can detect muons along with neutrons and gamma rays that are easy to fabricate.